This invention relates to method and apparatus for confirming shot peening coverage.
Shot peening has been common practice in the treatment of metal components to increase surface hardness and fatigue life. In shot peening, spherical shot is impacted on the surface of a component, thereby forming very small spherical dents on the surface. However, the process must be carefully controlled, because shot peening above and below a critical intensity results in a component having less than optimal properties.
Two critical shot peening perimeters are intensity and coverage. Peening intensity is a function of the kinetic energy of the shot impacted upon the surface of the component, which is a function of shot velocity and shot size. Commonly, shot is accelerated by using air pressure to force the shot through a peening nozzle which is directed at the surface undergoing peening. In order to confirm shot peening intensity, the almen strip process has been used for many years. Thin metal strips which deflect when undergoing peening are mounted in a special holder in which a critical section of the strip is unsupported. The strip thus installed on the holder is exposed to the shot stream under (as closely as possible) the same conditions as the component undergoing peening. After the strip has undergone peening for a predetermined time period, the strip is removed from the holder and the deflection measured, all according to known procedures. Accordingly, a series of almen strips are exposed to the shot stream for increasing time periods. When the deflection of the strips increases by no more than ten percent (10%) when the time is doubled, the intensity is said to be at saturation, and peening of the component parts may begin at this saturation intensity.
Peening coverage of a component is a factor of intensity, but may be affected by other factors. For example, since the goal of peening is to cover the surface being peened by small dents or dimples formed by the impact of the shot, the hardness of the component will affect the size of the dimples and hence coverage is also a factor of hardness. The peening nozzle which directs shot to the component is usually automatically moved at a predetermined stroke rate. If the nozzle becomes out of adjustment, or the stroke rate is somehow changed either through a machine malfunction or otherwise, coverage may also be affected, even though the common almen strip procedure indicates no change in intensity. Therefore, coverage can only be confirmed by an extremely laborious and highly skilled method of physically observing the surface of the article being peened, usually with the aid of a microscope, in order to confirm that the entire surface has been covered with the aforementioned dimples or dents. In mass production, the intensity of the peening equipment is periodically checked by the almen strip method, but no convenient method of determining coverage, other than the aforementioned manual examination, has been available.
One prior art method of establishing peening coverage is disclosed in US. Pat. No. 3,950,642, which discloses a test strip coated with a fluorescent dye which is peened and then compared to a control strip to determine coverage, since a greater percentage of the fluorescent dye will be removed from the strip as coverage increases. Another prior art method in common use is to mount a test strip on a component undergoing peening and then microscopically examine the test strip to establish coverage, by examining for dimples or dents or changes in physical characteristics of the strip.
According to one embodiment of the invention, a test strip having surface irregularities distributed over the surface of the strip is mounted on the surface of a component undergoing peening, which is then peened. The test strip is then removed and the surface compared with a standard to determine coverage. This may be done by physically comparing the test strip with an archived standard test strip, comparing the test strip with a paper copy of the test surface of a test strip, or (if the examiner has had sufficient experience) by an examiner viewing the test strip and making a judgment as to whether coverage is sufficient.
According to another embodiment of the invention, a test strip having multiple regions of surface irregularities of graduated depth is mounted on a surface of the component undergoing peening for which peening coverage is to be confirmed. The component, with the test strip attached thereto, then is exposed to the shot stream under production conditions. The strip is then removed and examined. Since the surface irregularities are of graduating depth, and since the strip has been selected according to a procedure hereinafter described, when the test strip has been exposed to shot at the desired coverage level, some of the irregularities will be obliterated while the deeper irregularities will remain. The test strip may also be compared with an archived standard test strip or a copy of an archived standard test strip.
Accordingly, coverage of shot peening is confirmed without the laborious inspection process of the prior art. It is contemplated that only a representative sample of the components undergoing peening will have coverage confirmed. (For example, one out of every one hundred parts undergoing peening).